United States
Environmental Protection
Agency
Environmental Monitoring
Systems Laboratory
Research Triangle Park, NC 27711-
Research and Development
EPA/600/S4-88/014 June 1988
&EPA Project Summary
A Review of Methods for
Remote Sensing of Atmospheric
Emissions from Stationary
Sources
Mark L Saeger, Cynthia K. Sokol, Susan J. Coffey, Robert S. Wright, William
E. Farthing, and Kim Baughman
This document provides a review
of several remote monitoring sys-
tems that can be used to detect the
presence of, or determine the
concentration of, pollutant emissions
in air. Remote sensing technologies
were categorized as passive or active
depending on the light source
employed in the system. The
description of techniques is organ-
ized into three sections:
commercially available instruments,
developing technologies, and po-
tential developments. A brief
discussion of practical consider-
ations related to the use of remote
sensing for air pollution emissions
monitoring is also presented.
This Project Summary was
developed by EPA's Environmental
Monitoring Systems Laboratory,
Research Triangle Park, NC, to
announce key findings of the
research project that is fully
documented in a separate report of
the same title (see Project Report
ordering information at back).
Introduction
The surveillance of air pollutant
source emissions is of concern to
regulators, process operators, process
engineers, air pollution control engineers,
and air pollution researchers. The
primary objective of this research was to
provide a summary of remote sensing
capabilities that have been used for, or
are applicable to, air pollutant emissions
monitoring. Many of these methods were
identified.
The remote sensing technologies
reviewed in this report are limited to
those that rely on the interaction of a light
source and some atmospheric property
to detect the presence of an atmospheric
constituent. Remote sensing technologies
have been divided into passive systems
and active systems. Passive systems use
some natural light source; in active
systems, a light source is included as a
part of the instrument design.
This report presents an overview of
remote sensing technologies and of the
physical and chemical principles that are
related to remote sensing. The specific
remote sensing instruments identified
during this effort are described. Practical
considerations related to the application
of methods for remote sensing of air
pollutant emissions are presented.
Techniques and Principles
The basic physical principles behind
many remote sensing techniques are
discussed in general terms. As
throughout the rest of this report, this
section discusses passive and active
techniques separately. Although passive
techniques are often simpler, more
mobile, and less expensive than active
techniques, their use is often restricted to
daylight hours and to good meteorol-
ogical conditions.
Passive techniques may operate in
either the emission mode or the
adsorption mode of molecular spec-
troscopy. In the emission mode, the
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target gas (e.g., a heated plume)
produces the radiation that is detected.
In the absorption mode, an external but
natural radiation source (e.g., sunlight)
interacts with the target gas and is
detected.
The most common light sources
being used for active techniques are
lasers. They are chosen for the high
power that they have available at specific
wavelengths of interest. However, with
this advantage comes a potential eye
hazard.
Summary of Remote Sensing
Technologies
Both commercially available
instruments and developing technologies
are discussed in this section. Table 1
summarizes the methods and their
developmental status. Such information
as cost, weight, and power requirements
are given for the commercially available
instruments. In many cases, specification
sheets for these instruments are
provided. There are more passive than
active instruments commercially
available. The relative cost of these two
classes of instruments appears to be a
determining factor in their availability.
Practical Considerations
Many applications of remote sensing
technologies to source emissions
monitoring have been discovered and
several have become commercial
ventures. In general, passive systems
have been applied to routine monitoring
of source emissions such as
smokestack-type emissions, and active
techniques have been applied more often
to area or line sources involving potential
accidental releases of explosive or
hazardous materials. The companies that
offer commercial pulsed laser measure-
ment systems also offer van-mounted
systems for lease at considerably less
cost than the purchase price.
Remote sensing of air pollutants can
be applied to a significant list of pollutant
species. Advancements and research
can provide capabilities for the
measurement of many more air
pollutants. Technologies exist that can be
used to measure almost any air pollutant
by remote sensing principles. The
investment of labor and money to
develop, build, and test a system can
often be significant and even prohibitive;
thus, careful planning is required for
projects concerned with emissions
monitoring by remote sensing. In many
instances, the benefits would justify the
potential costs, but every application
should be considered individually.
There are limitations to passive
remote sensing systems that rely on
natural radiation. One is the dependence
on favorable meteorological conditions.
There are wide variations of the light
source intensities in different locations,
during different seasons, and during
periods of the same day at the same
location. Compensation for the variations
often requires additional steps in the
signal processing and data reduction,
and short-term variations still contribute
to error. Another limitation of passive
remote sensors is the inability to
determine the amount of emissions as a
function of distance from the detector.
Thus, range-resolved spatial mapping
of the gas usually cannot be obtained.
Rather, the measurement is the integral
concentration of emissions along the
entire line of sight. The quantity
measured is optical depth, the product of
a concentration and a pathlength. These
limitations do not preclude the use of
passive devices for remote sensing of
source emissions. Passive sensors are
generally simpler, more convenient,
easier to operate, and easier to maintain
than active sensors. They are also
usually considerably less expensive than
active sensors.
Active systems have severe
limitations as well. They are often cum-
bersome, they sometimes require special
power supplies, and optics alignment can
involve regular maintenance and atten-
tion. Pulsed laser systems, used to
obtain range-resolved measurements,
require sophisticated signal-processing
electronics and significant data storage
capacity. The operation of the system,
data handling tasks, data interpretation,
and routine system maintenance often
require a skilled technician trained for the
task.
Advantages of active techniques are
related to extent of coverage either
spatially or temporally. The spatial
coverage offered by either computer-
controlled or mobile, mounted systems is
obvious. A sweep of an entire chemical
complex can be accomplished in several
minutes by using a computer-controlled
laser-based system. A mobile system
mounted on a truck or a van can define
the extent and characteristics of a widely
dispersed power plant plume in a short
time.
The temporal coverage provided by
continuous long-path systems makes
them well-suited for perimeter
monitoring and pipeline surveillance in
cases where hazardous materials can be
released inadvertently. Long-lasting
light sources and simple electronics that
focus on alarm level detection m
these systems capable of continu^
unattended operation for long period;
time. Often, lasers are not required
even desired for this type of applicat
and the simplicity that can be tolerj
brings the cost down to levels c<
petitive with*passive remote sens
systems.
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Table 1. Remote Sensing Methods for Source Emissions Measurements
and their Development Status
Method Class . Method Developmental Status
Passive Radiometers, heterodyne Developing technology
Radiometers, imaging Commercially available
Spectrometers Not applicable
Interferometers Commercially available
Correlation spectrometers, dispersive Commercially available
Correlation spectrometers, nondispersive Commercially available
Active Long-path infrared absorption Commercially available
Lidars, atmospheric (Mie) backscatter Commercially available
Udars, differential absorption Commercially available
Lidars, fluorescence Undeveloped
Lidars, Raman backscatter Commercially available
Lidars, Doppler Developing technology
Udars, wedge absorption Developing technology
Gas imaging Developing technology
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Mark L Saeger, Cynthia K. Sokol, Susan J. Coffey, and Robert S. Wright are
with Research Triangle Institute, Research Triangle Park, NC 27709.
William E. Farthing and Kim Baughman are with Southern Research
Institute, Birmingham, AL 55305
Roosevelt Rollins is the EPA Project Officer (see below).
The complete report, entitled "A Review of Methods for Remote Sensing of
Atmospheric Emissions from Stationary Sources," (Order No. PB 88-190
483/AS; Cost: $14.95, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Environmental Monitoring Systems Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
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